The present invention relates generally to the field of organometallic compounds. In particular, the present invention relates to alkyl Group VA metal compounds which are suitable for use as precursors for chemical vapor deposition.
Metal films may be deposited on surfaces, such as non-conductive surfaces, by a variety of means such as chemical vapor deposition (“CVD”), physical vapor deposition (“PVD”), and other epitaxial techniques such as liquid phase epitaxy (“LPE”), molecular beam epitaxy (“MBE”), and chemical beam epitaxy (“CBE”). Chemical vapor deposition processes, such as metalorganic chemical vapor deposition (“MOCVD”), deposit a metal layer by decomposing organometallic precursor compounds at elevated temperatures, i.e. above room temperature, either at atmospheric pressure or at reduced pressures.
A wide variety of metals may be deposited using such CVD or MOCVD processes. See, for example, Stringfellow, Organometallic Vapor Phase Epitaxy: Theory and Practice, Academic Press, 2nd Edition, 1999, for an overview of such processes. Organometallic compounds of arsenic, antimony, and bismuth are used to deposit epitaxial films in the semiconductor and related electronic industries. Epitaxial films such as gallium arsenide find applications in optoelectronic devices such as detectors, solar cells, light-emitting diodes (“LED's”), lasers and electronic switching devices such as field effect transistors (“FET's”) and high electron mobility FET's (“HEMT's”). Ternary arsenic alloys also exist such as gallium indium arsenide (“GaInAs”) and aluminum indium arsenide (“AlInAs”), which are more attractive than GaAs or aluminum gallium arsenide (“AlGaAs”) for the most powerful fiber optic systems operating in the 1.3 to 1.55 micron wavelength range. Gallium arsenide phosphide (“GaAsP”) is suitable for visible LED's and fiber optic emitters/detectors. Antimony and antimony alloy films are useful in fiber optics communication systems, particularly in the 1.3 and 1.55-micron regions. Antimony-containing semiconductor materials also have commercial applications including detection for seeker, night vision and surveillance devices (infrared detectors) and sources (LED's or lasers). A variety of binary, ternary and quaternary Group III/V semiconductor systems containing antimony have been evaluated for applications in infrared emitters and detectors operating in the 3 to 5 micron and 8 to 12 micron spectral ranges. These wavelength ranges are important since they are natural windows in the atmosphere for infrared transmission. Epitaxial antimony-based Group III/V semiconductors have potential applications in long wavelength detectors and high-speed electronic devices.
Arsine (“AsH3”) and phosphine (“PH3”) are attractive precursors for MOVPE since they provide arsenic and phosphorus along with hydrogen radicals that can scavenge any carbon-containing radicals generated during the MOVPE growth. However, the highly toxic nature of arsine and phosphine makes handling these gases in cylinders at high pressures dangerous. The threat of their rapid release in large quantities is serious and significantly high facility costs are often incurred to meet the appropriate safety requirements. Thus, there is a need to develop alternative Group VA hydride precursor compounds that are less hazardous than arsine and phosphine. Certain trialkyl Group VA metal compounds, such as trialkyl stibines, have been developed. However, such trialkyl compounds typically have low vapor pressures and higher decomposition temperatures. Such trialkyl compounds also result in carbon incorporation in the grown films. Monoalkyl Group VA dihydride compounds are excellent alternatives as they greatly reduce the amount of carbon incorporated in grown metal films.
For semiconductor and electronic device applications, these Group VA metal alkyls must be highly pure and be substantially free of detectable levels of both metallic impurities, such as silicon and zinc, as well as oxygenated impurities. Oxygenated impurities are typically present from the solvents used to prepare such organometallic compounds, and are also present from other adventitious sources of moisture or oxygen.
Methods of preparing monoalkyl arsines and phosphines by reacting arsine or phosphine gas with an alkene in the presence of a catalyst are known. Such methods are favored commercially as they require the handling of arsine gas or phosphine gas, which are both very toxic.
Grignard type syntheses of alkyl Group VA metal compounds are also known. For example, arsenic trihalide or phosphorus trihalide is reacted with an alkyl Grignard reagent to form a monoalkyl arsenic or monoalkyl phosphorus compound which is subsequently reduced to form monoalkyl arsine (RAsH2) or monoalkyl phosphine (RPH2). Such reactions are carried out in low boiling ethereal solvents, such as diethyl ether. While these Grignard reactions work well, the products are typically contaminated with residual ethereal solvent, and require extensive purification to remove the ethereal solvent. Even with such purification procedures, trace ethereal solvents remain in the monoalkyl arsines and phosphines. This remaining ethereal solvent, which is an oxygenated impurity, is undesirable for chemical vapor deposition processes.
Attempts have been made to reduce the amount of ethereal solvent in monoalkyl arsines and phosphines. For example, EP 839 817 A2 (Murakoshi et al.) discloses a method of preparing monoalkyl-arsines and -phosphines by first reacting an alkyl Grignard reagent with arsenic trihalide or phosphorus trihalide in diethyl ether to form an alkyl arsenic or phosphorus dihalide, removing the diethyl ether to leave a solid, adding diglyme (b.p. 160° C.) to the solid, reducing the alkyl arsenic or phosphorus dihalide with lithium aluminum hydride to form alkyl arsine or alkyl phosphine, and then distilling the alkyl arsine or alkyl phosphine. The distilled alkyl arsine and alkyl phosphine was found to contain ca. 9% or greater of diglyme. The alkyl arsine or phosphine product was then treated with a zeolite for seven days in order to reduce the amount of impurities. Such method is not practical commercially due to the extra cost of materials and long times required.
Accordingly, methods of preparing Group VA metal alkyl compounds in high yields and that are substantially free of both metallic and oxygenated impurities for use as precursor compounds for CVD are desired.